Among the nine Democratic presidential candidates who have already qualified for the September debate, five of them — former Vice President Joe Biden, Sen. Amy Klobuchar, D-Minn., former Texas congressman Beto O’Rourke, Mayor Pete Buttigieg of South Bend, Indiana, and Sen. Cory Booker, D-N.J. — have advocated, in one form or another, a health care public option. (Booker and O’Rourke, at least initially, want a buy-in for Medicaid, rather than Medicare.) Sen. Kamala Harris, D-Calif., calls her plan Medicare-for-all, but it offers Medicare as an option to other highly regulated private plans. That’s a 6-to-3 split opposed to a pure single-payer plan. (So much for the notion that the whole party has leaped far to the left.)

A key issue that separates the two camps is whether they are willing to keep some form of private insurance (Biden, Klobuchar, O’Rourke, Buttigieg, Booker, Harris); or whether they are pitching a true single-payer system with no room for private insurance (Sen. Bernie Sanders, I-Vt.; Sen. Elizabeth Warren, D-Mass., entrepreneur Andrew Yang).

Other issues that might distinguish the center-left from the left-left candidates might include a less-isolationist foreign policy and opposition to student loan forgiveness. The sheer number and expense of some candidates’ health plans (e.g., Warren) may underscore how progressive some of the progressives are. However, the fight over single-payer health care goes to the nub of the difference between the two groups of candidates: Who’s willing to shoot for ideological purity over practicality? Who understands there are unintended consequences from hypercentralization of power?

The way for the center-left contenders to demonstrate that they are the better alternative to the front-runner Biden is not, as we saw in the second debate, to go after the former vice president. He’s too popular and too sympathetic. Better simply to show oneself as the more effective, dynamic standard bearer of the center-left. And that requires the if-Biden-falters candidates from the center-left to launch a no-nonsense defense of the public option.

This comes at a time in which Sanders is sticking his chin out on Medicare-for-all, making it the defining issue, so it may be in contrasting themselves with him that center-left candidates make their mark. (Of the most progressive candidates, Sanders is also the most irritable and most likely to lose his cool. And, by process of elimination, he might be the best foil since going after Yang is punching down, and attacking the best debater on the stage, Warren, is awfully high-risk.) This doesn’t mean adopting the argument that Democrats want to stay out of Trump’s “socialist” strike zone, a perfectly legitimate point, but one which immediately puts them on defense. (Too timid! They’ll call you a socialist no matter what!)

No, what I am suggesting is a smart argument on the merits. Sanders and other Medicare-for-all supporters have always been fuzzy on how they are going to pay for it, how they are going to transition to a single-payer system, and how they are going to keep hospitals and doctors in business (especially in rural areas) with the low reimbursement rates. Maybe it is time for those who support keeping private insurance to make a robust argument against Medicare-for-all and for the public option. I know, it’s shocking that you’d want to debate on the merits, right?

In defending their own approach against the Medicare-for-all idea, the center-left candidates seeking to move up in case Biden falters will be showing how smart, aggressive and energetic they can be. The implicit message: Hey, moderates, you can have Biden’s moderation in a newer, flashier package.

It’s not clear whether primary voters want any infighting, so aggressively taking on Medicare-for-all is not without risks. But for a verbally dexterous, whip-smart candidate such as Buttigieg, or a candidate from a red state such as O’Rourke (who knows what worries more centrist Democrats), or a candidate pitching herself as the candidate from the heartland such as Klobuchar, it might be the only way to call attention to themselves and put them ahead of the rest for the center-left mantle if Biden were to falter. And if you are trailing the top four (Biden, Harris, Warren and Sanders), at some point, you’ve got to swing for the fences or go home.